Class 9 Science · Original EZT practice
Earth as a System: Energy, Matter, and Life (Class 9 Science Chapter 13): summary and practice
The Sun's energy and the cycling of matter keep life going on Earth. This chapter ties together ideas from across the book: how uneven heating drives winds and ocean currents, how water, carbon, nitrogen and oxygen cycle between air, land, oceans and living things, and how human activity is disturbing these processes. All questions are original EZT tutor practice following the chapter's own section order.
6 original worked examples, 6 fresh self-check questions and 4 mixed questions, with explained answers. These are not copied textbook exercises, an official chapter index or a full answer key. No affiliation with NCERT is claimed.
Official sources for topic and edition context, checked 22 October 2026:
1. Uneven heating of the Earth
Solar radiation is spread unevenly because of the Earth's shape and tilt. A surface's albedo tells how much sunlight it reflects - high-albedo surfaces like snow stay cooler, while low-albedo ones like black soil and ocean water absorb more and heat up more.
2. Uneven heating causes winds and ocean currents
Warm air rises over heated surfaces and cooler air flows in to replace it, producing local winds (valley and mountain breezes, land and sea breezes), planetary winds, and driving ocean currents.
3. Biogeochemical cycles - water and carbon
Water cycles through evaporation, condensation, precipitation, infiltration and flow back to the oceans. Carbon cycles between the atmosphere, oceans, living organisms and rocks, with the oceans holding most of the Earth's carbon.
4. Nitrogen and oxygen cycles; human impact
Nitrogen moves between the atmosphere, soil and living things through nitrogen-fixing microbes, fertilisers and decomposition; oxygen is exchanged through photosynthesis and respiration. Human actions - fossil fuel burning, deforestation, overuse of fertilisers - are disturbing these balances.
Choose, solve, check, then try a fresh question.
Before calculating, read the method prompt and name the condition that makes it useful. Write your own attempt. Open the steps only when you need them, and use the check to catch a wrong sign, unit or interpretation. Finish with the fresh question before reading its answer.
Compare albedo and warming
Choose the method: Low albedo absorbs more, heats more.
EZT practice question
Fresh snow has a high albedo while the open ocean has a low albedo. Which warms faster in sunshine, and why?
Show the worked solution
- Albedo measures how much sunlight a surface reflects.
- Snow reflects a large proportion back; the ocean reflects little and absorbs most of the incoming radiation.
- More absorbed energy means more heating.
Answer: The ocean. Low albedo means it reflects little and absorbs most of the incoming solar radiation, so it heats up more than high-albedo snow, which reflects a large proportion back.
Check: The same idea explains why black soil warms faster than pale sand in the same sunshine.
Watch for this: Do not assume whiter surfaces get hotter - a bright surface is reflecting the energy away.
Now try it yourself
The atmosphere consists mainly of nitrogen and oxygen. What are their approximate shares?
Check your answer
Nitrogen about 78%, oxygen about 21%, with small amounts of argon, carbon dioxide and water vapour.
The two main gases account for almost all of the air.
Explain uneven solar heating
Choose the method: Same sunlight, different spreading.
EZT practice question
Why do regions near the equator receive more solar heating than regions near the poles?
Show the worked solution
- The Earth is a sphere, so sunlight strikes the equator nearly straight on.
- Near the poles the same sunlight is spread over a larger surface area.
- It also passes through more atmosphere before reaching the ground.
Answer: Because of the Earth's shape - near the poles the same sunlight is spread over a larger surface area and passes through more atmosphere.
Check: Spreading the same energy over a larger area means less heating per unit area - that is the whole argument.
Watch for this: Do not say the poles are further from the Sun - the difference comes from the angle, not the distance.
Now try it yourself
What does a surface's albedo tell you?
Check your answer
How much sunlight it reflects.
High albedo reflects most; low albedo absorbs most.
Identify a valley breeze
Choose the method: Daytime: warm slopes pull air up.
EZT practice question
During the day in a Himalayan valley, which way does the air move - up the slopes or down? What is this wind called?
Show the worked solution
- Sun-facing slopes heat faster than the valley floor.
- Warm air over the slopes rises.
- Cooler valley air moves up to replace it - a valley breeze.
Answer: Up the slopes. Warm air over the heated slopes rises and cooler valley air moves up to replace it - a valley breeze. After sunset the flow reverses into a mountain breeze.
Check: The name follows where the air comes from: by day it comes from the valley, by night from the mountain.
Watch for this: Do not reverse day and night - the slopes heat by day and cool by night, and the wind follows the temperature.
Now try it yourself
Why do hill stations like Shimla and Dehradun feel a change in wind direction between day and night?
Check your answer
Day-time valley breezes blow up the heated slopes, while night-time mountain breezes bring cool, denser air down into the valley.
Heating and cooling of the slopes reverse the flow.
Link uneven heating to the monsoon
Choose the method: Hot land, low pressure, moist air in.
EZT practice question
How does uneven heating of land and ocean help drive the southwest monsoon over India?
Show the worked solution
- Land heats faster than the ocean in summer.
- The heated land creates low pressure over the subcontinent.
- Moisture-laden air from over the ocean flows in, bringing monsoon rain.
Answer: Land heats faster than the ocean, creating low pressure over the subcontinent that draws in moisture-laden air from over the ocean, bringing monsoon rain.
Check: It is the same rise-and-replace mechanism as a sea breeze, scaled up to a subcontinent.
Watch for this: Do not say the ocean heats faster - water warms and cools more slowly than land.
Now try it yourself
Besides winds, what else does uneven heating drive in the Earth system?
Check your answer
Ocean currents.
Moving warm and cool water is the ocean's version of the same process.
Read the Keeling curve
Choose the method: Subtract the old value from the new.
EZT practice question
The Keeling curve shows atmospheric CO2 rising from about 315 ppm in 1960 to about 420 ppm in 2025. By how many ppm has it risen, and what human activities caused this?
Show the worked solution
- Rise = new value minus old value.
- 420 - 315 = 105 ppm.
- The rise is mainly from burning fossil fuels and deforestation.
Answer: 105 ppm. The rise is mainly from burning fossil fuels and deforestation.
Check: 105 ppm over 315 ppm is about a third more - a large change for a finely balanced cycle.
Watch for this: Do not read the seasonal sawtooth dips as a falling trend - each year the curve ends higher than the last.
Now try it yourself
Out of the total quantity of global carbon, about how much is found in the oceans?
Check your answer
About 71%; the atmosphere holds only a very small fraction, roughly 1%.
The oceans are the Earth's main carbon reservoir.
Trace one change through the system
Choose the method: Follow the connections outward.
EZT practice question
How does clearing a large forest affect the Earth system beyond just losing trees?
Show the worked solution
- Fewer trees means less photosynthesis, so less CO2 is absorbed.
- Local rainfall can fall and surface albedo changes.
- River flow in the area changes too - one change propagates through connected processes.
Answer: It reduces photosynthesis, so less CO2 is absorbed; it can reduce local rainfall, alter surface albedo, and change river flow in the area - showing how one change propagates through connected processes.
Check: Each effect is a change in a different part of the system - air, water, land - which is what 'Earth as a system' means.
Watch for this: Do not stop at 'habitat loss' - the chapter asks you to trace the physical cycles, not only the biology.
Now try it yourself
What two processes keep oxygen moving between the air and living things?
Check your answer
Photosynthesis releases oxygen, while respiration (and decomposition) consumes it.
Release and consumption balance the oxygen cycle.
Try a mixed set
Choose a method or explanation before opening each answer.
1. What is albedo?
Show answer and reason
How much sunlight a surface reflects.
High albedo stays cooler; low albedo absorbs and heats more.
2. Valley breeze vs mountain breeze?
Show answer and reason
By day air rises up heated slopes (valley breeze); by night cool air sinks into the valley (mountain breeze).
Slopes heat and cool faster than the valley floor.
3. CO2 rise since 1960?
Show answer and reason
About 315 ppm to about 420 ppm - a rise of 105 ppm.
Mainly burning fossil fuels and deforestation.
4. Where is most of Earth's carbon?
Show answer and reason
In the oceans - about 71%.
The atmosphere holds only about 1%.
Try it yourself
Work these on paper before checking any notes. Answers are deliberately not shown here.
- The atmosphere consists mainly of nitrogen and oxygen. What are their approximate shares?
- How is climate change affecting the water cycle?
- Why is making nitrogen fertiliser energy intensive, and why does overuse matter?
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Questions about this practice page
- What is albedo?
- A measure of how much sunlight a surface reflects - high-albedo surfaces like snow reflect most sunlight and stay cooler, while low-albedo surfaces like ocean water absorb more and heat up.
- How much has atmospheric CO2 risen since 1960?
- From about 315 ppm to about 420 ppm (a rise of about 105 ppm, over a third), shown by the Keeling curve, mainly due to burning fossil fuels and deforestation.
- Where is most of the Earth's carbon stored?
- In the oceans - about 71% of the total global carbon; the atmosphere holds only about 1%.
- What is the difference between a valley breeze and a mountain breeze?
- By day, warm air rises over heated slopes and cooler valley air flows upward (valley breeze); after sunset the slopes cool faster and denser air flows down into the valley (mountain breeze).
- Are these the NCERT exercise questions for Class 9 Science Chapter 13?
- No. EZT pages are original practice following the chapter's section order - use them to prepare for the textbook exercises, not as a substitute for your book.